1. Field of the Invention
The present invention relates to an inter-ring connection method and device, and in particular to an inter-ring connection method and device upon establishing a network by a plurality of rings in an RPR (Resilient Packet Ring).
2. Description of the Related Art
A packet ring directly transferring therethrough an Ethernet (registered trademark) frame and the like is noticed in a Metro Area Network, and the RPR is standardized in the IEEE802.17 committee.
Hereinafter, the RPR technology will be schematically described.
RPR Ring
A ring using the RPR (hereinafter, occasionally referred to as RPR ring) is for transferring data by a dual ring arrangement in which two unidirectional rings (hereinafter, referred to as ringlets) are combined in the opposite directions to each other as shown in FIG. 13, and for providing a bandwidth sharing packet ring.
In the physical layer of the RPR, Ethernet (registered trademark) and SONET/SDH are adopted, so that 255 RPR stations (network devices composing the RPR; hereinafter occasionally referred to simply as stations) at the maximum are allowed to be provided for a single ring. Also, an IEEE802 48-bit MAC address (in conformity with the IEEE std 802-2002) is used for an identifier of the station.
Frame Format
FIG. 14 shows a format of a basic data frame. When the Ethernet (registered trademark) frame is relayed, an extended data frame format shown in FIG. 15 is used.
Broadcast
There are two main types for a transmission method of a broadcast frame in the RPR as follows, where a station having received the frame strips (discards) the frame when a ttl (time-to-live)=1 or sa (source address)=MAC address of the station itself:                Unidirectional Transmission (FIG. 16A)        
Frames are transmitted through only one ringlet. Upon transmitting the frames, “station number-1” is set to the ttl.                Bidirectional Transmission (FIG. 16B)        
Frames are transmitted through both ringlets. A cleave point is set on the ringlets and a hop number up to the cleave point is set for the ttl in each ringlet. There is not a rule concerning where the cleave point should be set. However, in most cases, the cleave point is set at the farthest point from a transmission station, i.e. at a position of (station number-1)/2 or station number/2. Also, at the time of a fault, a fault point assumes the cleave point.
It is to be noted that copy or add processing is performed at each station in both cases.
Topology Discovery
The RPR station detects a change of a network or a fault by always grasping a ring topology. Each station on the ring broadcasts a TP (topology and protection) frame that is a type of a control frame in a periodical manner where recommended value is 100 ms or when the status of the station or ring changes. FIG. 17 shows the format of the control frame, and FIG. 18 shows the format of the TP frame.
Also, by using an ATD (Attribute Discovery) frame shown in FIGS. 19A and 19B, attached information can be exchanged. The data unit of this ATD frame is in a TLV (Type-Length-Value) form as shown in FIGS. 19A and 19B.
A topology database as shown in FIG. 20 is constructed based on information collected from each station, so that a topology arrangement of an entire ring is recognized. This topology database is used for determining a transmission direction of a frame.
Fairness
This is a method of adjusting a best-effort bandwidth by using a fairness algorithm between adjacent stations in order to secure fairness of best-effort traffic between the stations.
Namely, when a congestion occurs as shown in FIG. 21, it becomes impossible for stations S2-S4 downstream from this congestion point to transmit best-effort (fairness eligible) traffic ((1) in FIG. 21). Therefore, until the congestion is resolved, a shaper of the best-effort traffic at each station is restrained. For the meantime, by a method of fairly assigning the bandwidth to all of the stations and redistributing the remainder if a usage rate does not reach 100%, the best-effort bandwidth between the stations is adjusted.
A shaping parameter for the best-effort traffic is calculated by the fairness algorithm, and transmitted, by a fairness frame of a format shown in FIG. 22, to an upstream station ((2) in FIG. 21).
Protection
By a ring protection means called steering and wrapping, an extremely high speed fault protection (avoidance) within 50 ms is guaranteed within a single ring.
Namely, when a fault section (between stations S3-S4) is found in a steering protection method shown in FIG. 23A, a source station S3 switches over (steers) a direction (ringlet) of transmitting traffic so that the traffic reaches a destination station S5 while avoiding the fault section, thereby realizing the protection.
On the other hand, when a fault section is found in a wrapping protection method shown in FIG. 23B, the traffic is looped back (loopbacks LB1 and LB2) while avoiding the fault section, thereby realizing the protection.
Inter-Ring Connection
As shown in FIG. 24, if a network is established by interconnecting a plurality of rings #1 and #2 with a single inter-ring connection device (occasionally referred to as interconnecting station) S4 (composed of a bridge S40, and RPR stations S41 and S42) or S5 (composed of a bridge S50, and RPR stations S51 and S52), a broadcast frame transferred across the rings ((1) in FIG. 24) can not be saved when a fault occurs in an interconnecting portion between the rings #1 and #2, Accordingly, in this example, it is required to provide both of the interconnecting stations S4 and S5 between the rings ((2) in FIG. 24).
However, if the rings are interconnected at a plurality of points in this way, loops are caused between the interconnecting stations S4 and S5 ((3) in FIG. 24) as shown by the thick lines, so that a broadcast storm occurs in such a case ((4) in FIG. 24) and a network is down ((5) in FIG. 24).
Therefore, a method of avoiding a loop by running a Spanning Tree Protocol (IEEE 802.1d) across the rings #1 and #2, as shown in e.g. FIG. 25, is general.
Meanwhile, as a well-known example of a method connecting a plurality of rings, there are a method of communicating between stations connecting a plurality of rings and determining working/protection (act/standby) (see e.g. patent document 1), and a method of establishing a virtual ring across a plurality of rings ((1) in FIG. 26) as shown in FIG. 26 (see e.g. patent document 2).
[Patent document 1] Japanese Patent Application Laid-open No. 2003-258822
[Patent document 2] International Publication No. WO 2004/095779
When a plurality of rings are interconnected with interconnecting stations as mentioned above, installing a redundancy protocol was required and a high-speed fault protection equivalent to the ring protection was difficult in the prior art using the Spanning Tree Protocol and the patent document 1 determining the working/protection.
Also, when a single ring is established by using a virtual ring in the same way as the patent document 2, a fault occurrence influenced the network composed of a plurality of rings ((2) in FIG. 26) in its entirety.